Landau-Devonshire theory [1] is a phenomenological theory of ferroelectric materials which are characterized by having a spontaneous polarization below the Curie temperature C T . Various properties such as the dielectric constant can be modelled with this theory. Here we pay particular attention to how the polarization profile can be calculated and then show how, via the Landau-Khalatinkov equations, the response to an incident electromagnetic field can be calculated. Obviously, this is just the sort of information relevant to filters design in the sense that it is essentially the response to an electromagnetic field that is important, especially the wave nature apparent at high enough frequencies.